Method Of Making A Sensor Package With Cooling Feature

ABSTRACT

A sensor device includes a first substrate of semiconductor material having opposing first and second surfaces, photodetectors configured to receive light impinging on the first surface, and first contact pads each exposed at both the first and second surfaces and electrically coupled to at least one of the photodetectors. A second substrate comprises opposing first and second surfaces, electrical circuits, a second contact pads each disposed at the first surface of the second substrate and electrically coupled to at least one of the electrical circuits, and a plurality of cooling channels formed as first trenches extending into the second surface of the second substrate but not reaching the first surface of the second substrate. The first substrate second surface is mounted to the second substrate first surface such that each of the first contact pads is electrically coupled to at least one of the second contact pads.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/290,522, filed Oct. 11, 2016, which is a divisional of U.S. patentapplication Ser. No. 14/551,262, filed Nov. 24, 2014, which claims thebenefit of U.S. Provisional Application No. 61/912,476, filed Dec. 5,2013, which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to packaging of microelectronic sensordevices such as photonic sensors or accelerometers in a single packagewith their ASIC processors in a manner that is compact yet providesimproved cooling capabilities.

BACKGROUND OF THE INVENTION

As the semiconductor industry pushes for more density and performance,IC stacking structures have become a prominent solution. However, the ICpackage in stacking structures tends to run much hotter.

A conventional chip stacking technique is disclosed in U.S. PatentPublication 2013/0280864, which stacks the IC chip on an interposer. Tosolve the thermal issues, a standalone heat sink is attached over thetop of the package. Attaching a heat sink over the semiconductor packagehas been a standard solution used for IC cooling. However, this solutionis bulky and is not viable for sensor packages because the sensor activearea needs to be exposed to the environment (i.e. for receiving what isbeing sensed—e.g. incoming light). Placing a heat sink over the packagewould block and seal away the sensor active area preventing its properoperation.

There is a need for a low profile technique for stacking IC chips suchas a sensor device over associated ASIC semiconductor wafer (e.g. thesensor's processor unit) which includes a cooling solution all within asingle package.

BRIEF SUMMARY OF THE INVENTION

The aforementioned problems and needs are addressed by a method offorming a sensor device comprises providing first, second and thirdsubstrates. The first substrate of semiconductor material comprisesopposing first and second surfaces, a plurality of photodetectorsconfigured to receive light impinging on the first surface, and aplurality of first contact pads each electrically coupled to at leastone of the plurality of photodetectors. The second substrate comprisesopposing first and second surfaces, electrical circuits, and a pluralityof second contact pads each electrically coupled to at least one of theelectrical circuits. The method includes forming a plurality of firsttrenches into the second surface of the second substrate but notreaching the first surface of the second substrate, and mounting thesecond surface of the first substrate to the first surface of the secondsubstrate. The third substrate comprises opposing first and secondsurfaces, a plurality of third contact pads disposed at the firstsurface of the third substrate, and a plurality of fourth contact padsdisposed at the first surface of the third substrate. The method furthercomprises mounting the first surface of the third substrate to thesecond surface of the second substrate such that each of the secondcontact pads is electrically coupled to at least one of the thirdcontact pads, and electrically connecting a plurality of wires betweenthe first contact pads and the fourth contact pads.

Other objects and features of the present invention will become apparentby a review of the specification, claims and appended figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-9 are side cross sectional views showing the steps in formingthe packaged sensor device with an integrated cooling solution.

FIGS. 10-13 are side cross sectional views showing the steps in formingan alternate embodiment of the packaged sensor device with an integratedcooling solution.

FIG. 14 is a side cross sectional view showing another alternateembodiment of the packaged sensor device with an integrated coolingsolution.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a low profile method and structure for stackinga sensor device over its processor unit, while providing a coolingsolution, all within a single package. FIGS. 1-9 illustrate the steps informing the packaged sensor device with integrated cooling solution.

The process begins by providing a backside illuminated sensor wafer 10,which is well known in the art. One example is shown in FIG. 1, whichincludes a substrate 12 with sensor active areas 14 containing photodetectors 16 at or near the substrate's front surface 18. The photodetectors 16 are configured to receive light through the back side (i.e.through the back surface 20) of the substrate 12 and generate anelectrical signal in response to that received light. The sensor wafer10 also includes supporting circuitry 22 and sensor pads 24 at the frontsurface 18 that are connected to the photo detectors 16 and/orsupporting circuitry 22 for providing the electrical signals from thephoto detectors 16 to the outside world. Multiple sensors (each with itsown photo detectors 16, supporting circuitry 22 and sensor pads 24) areformed on the same wafer 10 (two such sensors are shown in FIG. 1).

An insulation (passivation) layer 26 such as silicon dioxide (oxide) orsilicon nitride (nitride) is formed on the front surface 18 of substrate12. Preferably, the passivation layer 26 is made of silicon dioxide ofat least 0.5 μm in thickness. Silicon dioxide deposition can be ChemicalVapor Deposition (CVD), sputtering, or any another appropriatedeposition method(s). The portions of passivation layer 26 over thesensor pads 24 are selectively removed with appropriate photolithographymasking (i.e. photoresist deposition, mask exposure and selectiveremoval) and plasma etching techniques. If passivation layer 26 issilicon dioxide, then the etchant can be CF4, SF6, NF3 or any otherappropriate etchant. If passivation layer 26 is silicon nitride, thenthe etchant can be CF4, SF6, NF3, CHF3 or any other appropriate etchant.An interconnect 28 is then attached to each of the exposed sensor pads24. The interconnects 28 can be a Ball Grid Array (BGA), a polymer bump,a copper pillar or any other appropriate interconnect component that iswell known in the art. Copper pillar or BGA (as shown) are preferredchoices for interconnects 28. The resulting sensor wafer structure isshown in FIG. 2.

An ASIC wafer 30 is provided, as shown in FIG. 3. ASIC wafer 30 includesa substrate 32 that contains electrical circuits 34 which areelectrically connected to bond pads 36 disposed on the top surface 38 ofthe substrate 32. The ASIC wafer substrate 32 is preferably made ofsilicon. Multiple ASIC dies 40 are formed on the same substrate 32 (twosuch dies are generically represented in FIG. 3). The ASIC wafer 30 canhave a single layer of electrical circuits 34, or multiple layers ofelectrical circuits 34 within the substrate 32. Electrical circuits 34can be conductive traces, electrical devices, both, etc.

An insulation (passivation) layer 42 such as silicon dioxide or siliconnitride is formed over the top surface 38 of the ASIC wafer 30.Preferably, this passivation layer 42 is made of silicon dioxide with athickness of at least 0.5 μm. Silicon dioxide deposition can be PlasmaEnhanced Chemical Vapor Deposition (PECVD), Chemical Vapor Deposition(CVD), or any another appropriate deposition method(s). The portions ofpassivation layer 42 over the bond pads 36 are selectively removed withappropriate photolithography masking (i.e. photoresist deposition, maskexposure and selective removal) and plasma etching techniques. Ifpassivation layer 42 is silicon dioxide, then the etchant can be CF4,SF6, NF3 or any other appropriate etchant. If passivation layer 42 issilicon nitride, then the etchant can be CF4, SF6, NF3, CHF3 or anyother appropriate etchant. A supportive layer 44 is then formed overpassivation layer 42. Supportive layer 44 can be polymer or glass.Preferably, supportive layer 44 is a type of photo reactive liquidpolymer deposited over the passivation layer 42 by spray deposition.Portions of the supportive layer 44 over and adjacent to the bond pads36 are selectively removed, preferably using photolithography etching(i.e. the bond pads 36 are exposed as well as portions of passivationlayer 42 around the bond pads 36, leaving a stepping of layers). Theresulting structure ASIC wafer structure is shown in FIG. 4.

The sensor wafer 10 and the ASIC wafer 30 are bonded together (i.e.front surface 18 bonded to top surface 38) using thermal compression orthermal sonic techniques that are well known in the art. An optionallayer of adhesive can be deposited over the supportive layer 44 on theASIC wafer 30 by roller before bonding. After compression, correspondingones of the bond pads 36 and sensor pads 24 are electrically connectedby the corresponding interconnect 28. Silicon thinning can then beperformed by mechanical grinding, chemical mechanical polishing (CMP),wet etching, atmospheric downstream plasma (ADP), dry chemical etching(DCE), or a combination of aforementioned processes or any anotherappropriate silicon thinning method(s) applied to back surface 20 toreduce the thickness of substrate 12 (i.e. reduce the amount of siliconover the photo detectors 16). The resulting structure is shown in FIG.5.

An optional optical layer can be deposited over the active areas 14. Forexample, the optical layer can include light manipulation elements suchas color filters and microlenses 46. A protective layer 48 is depostiedover the active side of the sensor wafer 10 covering the active areas14. A preferred protective layer 48 is protective tape. Portions of theprotective tape 48 are selectively removed (e.g. using photolithography,a laser, etc.) thus exposing portions of the substrate 12 between theactive areas 14. An anisotropic dry etch is used to form trenches 50into the exposed surface of the substrate 12 between the active areas14. The enchant can be CF4, SF6, NF3, Cl2, CCl2F2 or any otherappropriate etchant. The trenches 50 extend down to and expose thesensor pads 24. Another passivation layer 52 is deposited on the backside of the sensor wafer 10. Preferably, passivation layer 52 is made ofsilicon dioxide with a thickness of at least 0.5 μm, using silicondioxide deposition such as Chemical Vapor Deposition (CVD), sputteringor any another appropriate deposition method(s). Portions of thepassivation layer 52 over the protective tape 48 and sensor pads 24 areremoved with appropriate photo lithography masking and plasma etchingtechniques that are well known in the art. If passivation 52 is silicondioxide, then etchant can be CF4, SF6, NF3 or any other appropriateetchant. If passivation 52 is silicon nitride, then etchant can be CF4,SF6, NF3, CHF3 or any other appropriate etchant. The resulting structureis shown in FIG. 6.

A layer of photoresist is deposited on the bottom surface of the ASICwafer substrate 32, and patterned via photolithography to exposeselective portions of substrate 32. The pattern formed in thephotoresist depends on the design of the cooling channels to be formed,and can have many numbers of variations depending on the preferreddesign specification. The pattern in the photoresist will dictate howthe ASIC wafer substrate is etched to increase its surface area thusincreasing its cooling capability. One preferred pattern is intersectingrows and columns of lines. An anisotropic dry etch is used to formtrenches 54 into the exposed portions of the bottom surface of the ASICwafer substrate 32. The enchant can be CF4, SF6, NF3, Cl2, CCl2F2 or anyother appropriate etchant. The walls of the trenches 54 can be verticalor can be tapered. The trenches 54 form cooling channels that extendinto the bottom surface of the substrate 32. After the photoresist isstripped, an optional diffusion material 56 such as silicon nitride canbe formed on the bottom surface of substrate 32 (including in trenches50). This can be followed by forming an optional highly thermallyconductive material(s) 58 on the bottom surface of substrate 32(including in trenches 50). The highly thermally conductive materiallayer 58 formed on the diffusion material layer 56 is preferably one ormore metals (preferably both titanium and copper), which are depositedby Physical Vapor Deposition (PVD). The resulting structure is shown inFIG. 7.

The cooling channels formed by trenches 54 can be transformed intocooling tunnels by covering them with a substrate 60. The substrate 60could be any appropriate structure or thin film bonded to the bottomsurface of the ASIC wafer substrate 32. For example, the substrate 60could be die attached tape, a metallic foil or a silicon wafer. Thesecooling tunnels can be used to direct air flow to the sides of thepackage for heat dissipation. Wafer level dicing/singulation ofcomponents can be done with mechnical blade dicing equipment, lasercutting or any other apporiate processes along scribe lines betweenactive areas 14, resulting in separate sensor packages each containing asensor wafer die with its own active area, as illustrated in FIG. 8.

The individual sensor packages can be mounted to a host device such asan interposer, a printed circuit board or flex printed circuit board. Asshown in FIG. 9, the sensor package is connected to a printed circuitboard (PCB) 64 by interconnects 66 that make an electrical connectionbetween the sensor wafer bond pads 24 and bond pads 68 of the PCB 64.The PCB 64 preferably includes an aperture or window 70 that allows thesensor's active area to be exposed to incoming light. The electricalinterconnects 66 between the host and sensor package could be a ballgrid array, copper pillars, adhesive bumps or any other bondingtechniques that are appropriate. An optional underfill can be depositedaround the sensor package after it is mounted. FIG. 9 shows the finalstructure after the protective tape is removed. Air flowing through thecooling tunnels 54 efficiently removes heat from the package(originating from the sensor wafer die 30 and flowing to the ASIC waferdie 10), given the expanded surface area of the bottom surface of theASIC die 10 because of the cooling tunnels.

FIGS. 10-13 illustrate the steps in forming an alternate embodiment ofthe packaged sensor device with integrated cooling solution. The processbegins with the same processing steps as described above with respect toFIGS. 1-6, except without passivation layers 26 and 42, withoutinterconnects 28, and without patterning the supportive layer 44, sothat wafers 10 and 30 are bonded together without sensor pads 24 beingelectrically connected to bond pads 36, as shown in FIG. 10.

Trenches 54 are formed into the bottom surface of the ASIC wafersubstrate 32 as described above. A layer of photoresist is thendeposited on the bottom surface of the ASIC wafer substrate 32, andpatterned via photolighography to remove those portions of thephotoresist between the sets of cooling channels (i.e. those portionsnear the scribe lines), leaving portions of the bottom surface of theASIC wafer exposed. An anisotropic dry etch is used to form trenches 74in the exposed portions of the ASIC wafer bottom surface. The enchantcan be CF4, SF6, NF3, Cl2, CCl2F2 or any other appropriate etchant. Thetrenches 74 extend to and expose bond pads 36. The walls of the trenches74 can be vertical or tapered. The photoresist is then removed,resulting in the structure shown in FIG. 11.

The diffusion layer 56 and metal layer 58 are formed on the bottomsurface of substrate 32 (including inside trenches 54) as describedabove. Diffusion and metal layers 56/58 are selectively removed by theuse of lithographic masking and plasma etching to expose the ASIC waferbond pads 36. An insulation layer 76 is formed around bond pads 36 toprotect against an electrical shorts to the conducive metal materials.The insulation layer 76 can be solder mask that is selectively formedaround the bond pads 36. Preferably the insulation is deposited by spraycoating, followed by a lithographic process to selectively remove theinsulation except for around the bond pads 36. The resulting structureis shown in FIG. 12.

Wafer level dicing/singulation of components is performed (e.g. withmechnical blade dicing equipment, laser cutting or any other apporiateprocesses) along scribe lines between active areas, resulting inseparate sensor packages each containing a sensor wafer die with its ownactive area. The individual sensor packages can be mounted to a hostdevice such as a printed circuit board (PCB) or a flex printed circuitboard. The host (shown as a PCB 78) preferably includes an aperture,trench or cavity 80 in which the package at least partially sits.Wirebonds 82 are used to connect the sensor pads 24 to conductive pads84 on the host PCB 78. The bond pads 36 of the ASIC die 30 are connectedto other conductive pads 84 of host PCB 78 through ball grid arrayinterconnects 86 (or any other flipchip interconnection). Finally, theprotective tape is removed thus exposing the sensor active area,resulting in the structure shown in FIG. 13. Air flowing through thecooling tunnels 54 efficiently removes heat from the package(originating from the sensor wafer die 30 and flowing to the ASIC waferdie 10), given the expanded surface area of the bottom surface of theASIC die 10 because of the cooling tunnels.

FIG. 14 illustrates another alternate embodiment, in which PCB 78includes a through hole 90 instead of cavity 80 in which the package atleast partially sits. Substrate 60 as described above can be mounted tothe bottom surface of the ASIC wafer substrate 32 so that coolingtrenches 54 are cooling tunnels.

It is to be understood that the present invention is not limited to theembodiment(s) described above and illustrated herein, but encompassesany and all variations falling within the scope of the appended claims.For example, references to the present invention herein are not intendedto limit the scope of any claim or claim term, but instead merely makereference to one or more features that may be covered by one or more ofthe claims. Materials, processes and numerical examples described aboveare exemplary only, and should not be deemed to limit the claims.Further, as is apparent from the claims and specification, not allmethod steps need be performed in the exact order illustrated orclaimed, but rather in any order that allows the proper formation of thepackaged semiconductor device of the present invention. Lastly, singlelayers of material could be formed as multiple layers of such or similarmaterials, and vice versa.

It should be noted that, as used herein, the terms “over” and “on” bothinclusively include “directly on” (no intermediate materials, elementsor space disposed therebetween) and “indirectly on” (intermediatematerials, elements or space disposed therebetween). Likewise, the term“adjacent” includes “directly adjacent” (no intermediate materials,elements or space disposed therebetween) and “indirectly adjacent”(intermediate materials, elements or space disposed there between),“mounted to” includes “directly mounted to” (no intermediate materials,elements or space disposed there between) and “indirectly mounted to”(intermediate materials, elements or spaced disposed there between), and“electrically coupled” includes “directly electrically coupled to” (nointermediate materials or elements there between that electricallyconnect the elements together) and “indirectly electrically coupled to”(intermediate materials or elements there between that electricallyconnect the elements together). For example, forming an element “over asubstrate” can include forming the element directly on the substratewith no intermediate materials/elements therebetween, as well as formingthe element indirectly on the substrate with one or more intermediatematerials/elements therebetween.

What is claimed is:
 1. A method of forming a sensor device, comprising:providing a first substrate of semiconductor material that comprises:opposing first and second surfaces, a plurality of photodetectorsconfigured to receive light impinging on the first surface, and aplurality of first contact pads each electrically coupled to at leastone of the plurality of photodetectors; providing a second substratethat comprises: opposing first and second surfaces, electrical circuits,and a plurality of second contact pads each electrically coupled to atleast one of the electrical circuits; forming a plurality of firsttrenches into the second surface of the second substrate but notreaching the first surface of the second substrate; mounting the secondsurface of the first substrate to the first surface of the secondsubstrate; providing a third substrate that comprises: opposing firstand second surfaces, a plurality of third contact pads disposed at thefirst surface of the third substrate, and a plurality of fourth contactpads disposed at the first surface of the third substrate; mounting thefirst surface of the third substrate to the second surface of the secondsubstrate such that each of the second contact pads is electricallycoupled to at least one of the third contact pads; and electricallyconnecting a plurality of wires between the first contact pads and thefourth contact pads.
 2. The method of claim 1, further comprising:forming a cavity into the first surface of the third substrate, whereinat least a portion of the second substrate is disposed in the cavity. 3.The method of claim 1, further comprising: forming a through hole thatextends between the first and second surfaces of the third substrate,wherein at least a portion of the second substrate is disposed in thethrough hole.
 4. The method of claim 3, further comprising: mounting afourth substrate to the second surface of the second substrate, whereinthe fourth substrate covers the first trenches.
 5. The method of claim1, further comprising: forming one or more second trenches into thefirst surface of the first substrate, wherein the first plurality ofcontact pads are disposed in the one or more second trenches.
 6. Themethod of claim 1, further comprising: forming one or more secondtrenches into the second surface of the second substrate, wherein thesecond plurality of contact pads are disposed in the one or more secondtrenches.